The present invention relates generally to gas turbine engines, and, more specifically, to wide chord fan blades therein.
A turbofan gas turbine engine includes a row of fan blades powered by a low pressure turbine (LPT). Air initially enters the engine through the fan and an inner portion thereof enters a compressor which pressurizes the air for mixing with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through a high pressure turbine (HPT) which extracts energy for powering the compressor. The combustion gases then flow through the LPT which extracts additional energy therefrom for powering the fan. The remaining outer portion of the air flowing through the fan is discharged from the engine for producing thrust to power an aircraft in flight.
A fan blade includes a dovetail at its radially inner end which is trapped in a complementary dovetail slot in the perimeter of a rotor disk. An airfoil is attached to the dovetail by a structural shank. Platforms may be joined integrally with the blade or separately attached between adjacent blades for providing a radially inner flowpath boundary for the fan air, with the platform being radially located atop the shank at a radially inner root of the airfoil.
The airfoil extends radially outwardly to an opposite tip, and has a forward or leading edge and an axially opposite aft or trailing edge collectively defining the perimeter of the airfoil. The airfoil has a generally concave or pressure first side and a circumferentially opposite convex or suction second side. The airfoil has a span or longitudinal axis extending in the radial direction from the centerline of the rotor disk to which it is attached, and various chords extending generally axially between the leading to trailing edges. The airfoil typically twists from its root to its tip for maximizing aerodynamic performance.
Wide chord fan blades have a relatively low aspect ratio which is its span to chord ratio and are relatively heavy when formed as solid metal parts. Weight reduction is typically obtained by using high strength superalloy materials such as those including Titanium. However, as engines grow larger in size the corresponding fan blades increase in size and weight, and increase the difficulty in achieving a suitable life therefor under the high centrifugal loads generated during operation.
In separate developments, all composite fan blades have been designed for reducing weight while providing acceptable performance in the gas turbine engine environment. A typical composite blade includes several layers of structural fibers, such as graphite, embedded in a suitable matrix, such as epoxy, for tailoring blade strength in a lightweight structure. Composite blades require a complex manufacturing process and are expensive to produce.
Hybrid blades are also being developed which are primarily metal, such as Titanium, having suitable pockets therein for reducing weight, with the pockets being filled with a suitable filler material for completing the required aerodynamic profile of the airfoil. However, pockets in an otherwise structural airfoil reduce the stiffness thereof, or the corresponding moments of inertia, and thus create an additional problem in vibratory performance and foreign object damage (FOD) resistance.
More specifically, during operation a fan blade is subject to centrifugal force, aerodynamic force, and vibratory stimuli due to the rotation of the fan blades over the various operating speeds of the engine. A fan blade has various modes of resonant vibration due to the various excitation forces occurring during engine operation. A fan blade is basically cantilevered from its rotor disk and therefore may bend or flex generally in the circumferential direction in fundamental and higher order modes of flexure or flex. The airfoil is also subject to fundamental and higher order torsional modes of vibration which occur by twisting around the airfoil span axis. The flex and torsion modes of vibration may also be coupled together further increasing the difficulty of blade design.
Hybrid blades which include weight lightening pockets therein are also subject to local panel modes of vibration due to the remaining thin metal at the base of the pockets which may separately vibrate. In addition to these various modes of vibration of the individual blades, the full row of blades on a rotor disk may vibrate collectively in group modes.
Although hybrid blades being developed allow a substantial reduction in blade weight, the open ended pockets therein necessarily decrease both the bending and torsional stiffnesses, or moments of inertia, of the airfoil which adversely affects the various vibration modes. For example, the pockets reduce bending stiffness and may correspondingly lower the resonant frequency of the fundamental flex mode. This in turn decreases the frequency margin between the fundamental blade vibratory mode and the conventional 1/rev fundamental excitation frequency of the engine. The smaller the frequency margin, the greater is the excitation response and resulting vibratory displacement and stress, which may be reduced by suitable damping. The pockets also decrease the torsional stiffness of the blade which leads to reduction in frequency margin between torsion modes and adjacent flex modes, for example. This too may lead to undesirable blade excitation during operation from aerodynamic excitation forces.
The filler material is preferably viscoelastic for introducing damping to reduce vibratory response of the airfoil during engine operation. However, relatively large pockets in the airfoil do not promote significant shear strain in the filler since loads are carried therearound in the metal portions. This is especially significant for the torsional modes which effect a different response pattern than the flex modes.
Furthermore, since the fan blades are the first rotating structure in a gas turbine engine which receives intake air, they are also subject to foreign object damage (FOD), due to birds strike for example. Typical fan blades are therefore also designed to have suitable FOD strength, with flexibility at the leading edge region of the blade for withstanding a bird strike with little or no permanent damage thereto. The pockets being developed for hybrid blades necessarily decrease the stiffness of the airfoil aft of the leading edge thusly decreasing the ability of the airfoil to withstand foreign object damage.
Accordingly, it is desired to provide a hybrid fan blade having improved damping and FOD resistance.